1. Field of the Invention
This invention relates to a two wheel/four wheel drive change over apparatus for four-wheel drive vehicles.
2. Prior Art
A prior art two wheel/four wheel drive change over apparatus for four-wheel drive vehicles is disclosed in Japanese utility model application laid-open publication No. 58(1983)-2121, the disclosure of which is incorporated herein by reference. That document discloses a two wheel drive mode in which rotational power is transmitted from a transmission directly to either the front or rear wheels, and a four wheel drive mode wherein rotational power is transmitted to both the front and rear wheels by engagement of a transfer clutch. According to this prior art, there is provided an actuator which includes two chambers divided by a diaphragm. Each chamber is connected to a vacuum passage through an electromagnetic valve, and an engine vacuum supplied to both chambers is controlled by the electromagnetic valve, thereby operating the actuator. Since, in the prior art apparatus, the engine vacuum is introduced into the actuator directly from the engine, the fluctuation of the engine vacuum has a great influence on the apparatus, whereby the apparatus can be damaged by a change in the engine driving conditions. The engine vacuum is not generated during a full-open position of a throttle valve. Since the engine vacuum is generated during a constant driving or idling driving condition, only at this time can the supply of the engine vacuum be changed over.